The damaging effects of sunlight on human skin have been observed since time immemorial and many remedies have been proposed to protect the skin from this damage.
In general terms, harmful ultra-violet (UV) rays, particularly those originating from sunlight, which penetrate the upper atmosphere and reach the earth's surface, can be classified into:
i. the energy-rich UV-B rays (290-320 nm wavelength) which possess an intense physiopathological activity on the skin; these are absorbed just above the dermis and they are responsible for erythema and skin pigmentation, and
ii. UV-A rays (320-400 nm wavelength) which penetrate deeper into the skin (to the dermis and beyond). Their energy is much lower and the photobiological effects they cause are much more long term in nature, for example, they accelerate skin ageing.
Certain organic substances (sunscreens) whose molecules absorb the harmful ultra-violet rays have been proposed for use in mitigating the deleterious effects of ultra-violet radiation.
Some of these substances absorb more effectively in UV-A range thereby providing filtering of UV radiation in this range, while others are more effective in the UV-B range.
A common problem exists, however, whatever the choice of organic sunscreen, for protection from whichever wavelength of ultra-violet radiation, and this is that physiological damage to the body can occur, following topical application of these sunscreens in quantities necessary to provide effective filtering of harmful ultra-violet radiation. Even those organic sunscreens that are believed to be safe to use in this way, necessarily have safety limits imposed, based on the quantity applied to the skin, which can result in only moderate to poor protection from harmful ultra-violet radiation.
Certain inorganic substances have also been proposed for use as sunscreens which physically block exposure of the skin to ultra-violet rays. Notable of these is titanium dioxide having a very small particle size This grade of titanium dioxide, designated ultrafine TiO.sub.2, affords a good degree of sun blocking potential without the unacceptable skin whitening experienced with the normal pigmentary grade (particle size &gt;300 nm). For example, in DE-A-3824999 (The Boots Company PLC), it is proposed to use titanium dioxide with a mean primary particle size of &lt;100 nm in a water-in-oil emulsion as a sunscreen preparation. This reference also suggests that organic sunscreen agents, such as p-aminobenzoic acid and esters thereof, methoxycinnamate, benzophenone, dibenzoylmethanes or salicylates can also be included to improve protection.
In spite of this, and other prior proposals, there still exists a need for a highly efficient and thoroughly safe sun protection composition which has a wide spectrum of protection (i.e. both UV-A and UV-B) in the UV region.